TLT: What specific molecular mechanisms govern the adhesion of grease to metal surfaces, and how do these mechanisms influence additive performance?
Bonta: The adhesive behavior of greases is typically governed by a combination of mechanical and chemical interactions with the metal surface. In mechanical adhesion, grease penetrates the microscopic roughness and asperities present on the substrate. Once embedded, the grease’s thickener network or polymeric additives can act as an anchor, linking the bulk grease structure to the surface and helping retain the material in place. Chemical adhesion occurs when components of the grease, such as the base oil, thickener, polymer additives and other formulation constituents interact with the metal surface through weak intermolecular forces, including van der Waals interactions, dipole–dipole forces and in some cases hydrogen bonding. Together, these mechanisms provide formulators with multiple pathways to tune and enhance the adhesion of greases to metal surfaces.

These mechanisms must be carefully considered during grease formulation. When certain molecules interact strongly with the metal surface, they can interfere with the performance of additives that also rely on surface adsorption to function. This is particularly important for additives that operate under low-energy conditions, where effective performance depends on their ability to adsorb directly to the metal surface and form protective films. Competition for surface sites can therefore reduce additive effectiveness. As a result, extensive testing is often required to ensure that the overall formulation functions as intended and that interactions among components do not compromise the desired performance.

TLT: How do different thickeners, such as lithium or calcium-based ones, affect the distribution and activation of grease additives at the molecular level?
Bonta: Different grease thickeners influence additive behavior primarily through polarity, surface affinity and microstructure. Lithium soaps typically form fibrous networks with moderate polarity, allowing many additives to remain largely dissolved in the base oil and migrate relatively freely to metal surfaces where they can adsorb and activate. On the other hand, systems such as calcium sulfonate thickeners are more polar and can interact more strongly with additive molecules. This can alter additive distribution by either promoting association with the thickener structure or competing for adsorption sites on the metal surface. As a result, the thickener type can influence how efficiently additives reach the tribological interface and form protective films, making thickener–additive compatibility an important consideration during grease formulation.

TLT: What role does the chemistry of the metal surface (e.g., oxide layers or roughness) play in determining the effectiveness of grease additives like antiwear agents or extreme pressure additives?
Bonta: The chemistry of the metal surface strongly influences how effectively grease additives can function because most antiwear and extreme pressure additives operate by reacting with or adsorbing onto the surface. Oxide layers often provide reactive sites that allow additives such as ZDDP, sulfur-phosphorus compounds or other chemistries to form protective tribofilms. If the oxide layer chemistry changes, the rate and stability of film formation can change as well. Surface roughness also plays a role in affecting the real contact area and the ability of grease and additives to remain in the contact zone. Rougher surfaces generate high local temperatures and stresses that tend to drive additive reactions forward to generate tribofilms, but excessive roughness may increase stress beyond useful levels, accelerate film removal and lead to premature component failures. Overall, both surface chemistry and topography help determine how quickly and effectively protective films form and persist during operation.

TLT: How does rheology impact your work?
Bonta: All lubricants, especially greases, have the majority of their functional behavior dictated by their rheology. Because greases are structured, semi-solid materials, their ability to flow, deform and recover their structure directly controls how they perform in an application. In my work, rheology is essential for understanding properties such as yield stress, structural stability and viscoelastic response, which determines whether a grease will stay in place, release oil when needed and maintain a lubricating film under mechanical stress. By combining rheological measurements with traditional performance testing, we can better understand how formulation choices, such as thickener type, base oil viscosity and additives affect real-world performance. Ultimately, rheology provides a powerful tool for both developing new grease technologies and diagnosing performance issues in existing formulations.

TLT: Are there emerging techniques or technologies being used to visualize or model the molecular interactions between grease, additives and metal surfaces? If so, what insights have they provided?
Bonta: Visualizing grease structure at the microscopic level has historically been challenging, but several techniques have provided valuable insight. Traditionally, scanning electron microscopy (SEM) has been used to produce detailed images of grease microstructure. However, this approach often requires extensive sample preparation, such as solvent washing or drying that can disturb or degrade the native structure of the grease. More recently, techniques such as atomic force microscopy (AFM) and confocal fluorescence microscopy (CFM) have enabled researchers to observe grease structures with less disruption, offering a clearer view of their natural configurations.

At the molecular and surface level, a variety of analytical techniques are used to understand the chemical interactions that occur during lubrication. Methods such as scanning electron microscopy–energy-dispersive X-ray spectroscopy (SEM–EDS) and X-ray photoelectron spectroscopy (XPS) allow researchers to analyze metal surfaces after contact with lubricants and identify the elemental composition of deposited tribofilms. Complementary techniques, including Raman spectroscopy, can provide information about the nature of chemical bonds present on the surface, helping to reveal changes in surface chemistry that occur during lubrication. Together, these tools provide a multi-scale view of grease structure and surface interactions, from microstructure to molecular-level chemistry.

TLT: What is the most fun grease experiment you have done related to your research, and what did you learn from it?
Bonta: Some of my favorite experiments are the ones that require developing a test approach on the fly to evaluate performance in ways that are not covered by standard methods. In one case, I was tasked with designing a grease for an application that is typically lubricated with oil. The mechanism, however, was isolated and difficult to service, so a grease solution would have been ideal, provided it could operate effectively under the required conditions. The challenge was that the device ran at much higher speeds than greases are normally used in.

To evaluate whether a grease could succeed in this environment, we modified the operating conditions of a high-speed bearing test machine to more closely replicate the actual application. By adapting the test in this way, we were able to generate meaningful data that guided the formulation strategy and helped identify the most promising grease design. Experiences like this highlight how valuable flexibility and creative problem-solving can be when addressing tribological challenges in real-world applications.

You can reach Jacob Bonta at jacob@profluidllc.com.